The TS9DX FLEXI 4x2 starts with the same chip used in both of Robert Keeley's other modded Tube Screamers (the Baked Mod and Mod Plus): a Texas Instruments RC4558 chip, as used in original TS808 Tube Screamers. Metal film resistors are used for lo...

Steve's setup:
"The guitar in line goes into the Wah Wah on the board - either the Dunlop automatic one or the Bad Horsie (he switches between the two). From there it goes to the two distortion boxes, the DS-1 and the Keely modded TS-10, then the EvH flanger and from there it goes back into the front of the amp.
Note the error: He says TS-10 but as one can see in the photos here:
http://www.vai.com/minisites/tn/photo.html
When you click over the Tube Screamer photo you see it's a TS9DX with its 4 knobs and below it states:
"Steve's Keely modded Ibanez TS9DX"
In Keely's site it is stated Steve Vai uses this mod (click Artists):
https://robertkeeley.com/product/ts-9dx-flexi-4x2-mod/

Description

The TS9DX FLEXI 4x2 starts with the same chip used in both of Robert Keeley's other modded Tube Screamers (the Baked Mod and Mod Plus): a Texas Instruments RC4558 chip, as used in original TS808 Tube Screamers. Metal film resistors are used for low-noise operation, while stacked metal film capacitors and increased input impedance deliver a crisper, more lively tone that's incredibly transparent.

The TS9DX FLEXI 4x2 Tube Screamer mods don't stop there, however. in addition to standard Drive, Level, and Tone knobs, it has a four-way rotary "Turbo" knob for different overdriven tones on the fly. Combine that with the 2-way gain switch and you've got eight tube-amp like tones at your disposal.